Data storage systems, whether magnetic or optical, typically store the data in the form of transitions between two stable magnetic or optical states. The data is retrieved by a read head which typically senses the magnetic transitions or the two optical states by providing an analog pulse-type signal, which is amplified and filtered. The analog signal typically comprises positive-going and negative-going alternating pulses or positive-going and negative-going alternating edges. The data is represented by the location, or the timing, of the pulses. The peak is normally the best indicator of the center of the pulse for magnetic tape data and for pulse position modulation (PPM) optical data, and is therefore used to detect the data. Optical pulse width modulation data (PWM) is represented by the length of a pulse, the positive-going "front" edge of the pulse comprising a "1", the negative-going "back" edge of the pulse comprising a "1", and each clock time with no pulse edge comprises a "0".
The analog signal is sampled and converted into a digital signal by an analog to digital (A/D) converter. The digital samples are then utilized for processing to determine the presence of binary "ones" and "zeroes" in the sensed data. In the case of magnetic media, each peak, whether positive or negative, represents a transition and is a binary "one". Each clock cycle between transitions, that is without a transition, is a binary "zero". In the case of optical media, either pulse position modulation or pulse width modulation is used. In pulse position modulation (PPM), a full optical laser pulse comprises a single binary "one", and is represented by only the positive-going peak and not by the negative-going peak. Pulse width modulation (PWM) is represented by the length of a pulse more analogous to magnetics, where a binary "one" is represented by the beginning of the optical laser pulse, a positive-going edge, and by the end of the optical laser pulse, a negative-going edge.
The peaks representing the magnetic transitions or PPM pulses, and edges representing the PWM edges are typically identified and then must be compared to a threshold or threshold envelope to "qualify" the identified peak or edge as having sufficient amplitude that it is data and not noise. The threshold value is designed to be a fraction of the nominal peak value, and is typically set at about 50 percent of the nominal peak. A fixed system may miss peaks or interpret noise as a peak as a result of signal amplitude variation or baseline wander. Sources of signal amplitude variation and baseline wander may occur in many ways. For example, in a magnetic tape system, variations in signal amplitude may occur as a result of the magnetic tape lifting off the read head due to particles or artifacts located on the magnetic tape. Vibrations and other conditions may also cause the magnetic tape to lift off the read head. In an magneto-resistive (MR) head, debris or an asperity on the tape may strike the head, resulting in a thermal spike which causes an additive offset to the signal from a change in the resistance.
Hence, some magnetic signal peak detectors have circuits for providing an adjustable threshold value.
An example of an adjustable threshold circuit is coassigned U.S. Pat. No. 5,363,100, Bailey, et al., issued Nov. 8, 1994. The patent describes a hybrid analog-digital method using pattern lengths to select gains and adjust currents for generating a tracking threshold.
An example of a digital adjustable threshold circuit is coassigned U.S. Pat. No. 5,530,601, Hutchins et al., issued Jun. 25, 1996. The patent shows that a threshold value for a selected peak is established utilizing a prior threshold value associated with a prior peak set off by a number of spaces. The circuit thus filters the truncated waveform with a given delay to generate a tracking threshold. The tracking threshold is suitable for a single. media type and single type of data to reduce the errors in peak detection which occur as a result of signal amplitude variations.
Removable media, whether magnetic or optical, are subject to variability. The variability may comprise inconsistencies between manufacturers of the media. The variability may comprise the use of either PWM or PPM media in the same drive. Two types of recording are now being proposed for use on the same medium, or the use of two recording media on the same substrate, allowing for variability within the same medium. The variability may also arise from modernization of the media. The variability of the media most often appears in the variability of the transitions and therefore the variation in amplitudes and offset of the peaks of the data to be detected.
In addition, recording codes can have variability. Most recording methods, such as optics or magnetics, increase the apparent data capacity by the use of more advanced recording codes which have increased length and which incorporate long strings of "zeroes". The conventional threshold tracking circuits, above, would have to be altered to change the time constants to fit the longer codes so as to avoid having the threshold amplitude decrease to the level that, without the alteration, noise would be detected as data. On the other hand, should data stop, the tracking circuit should not get "hung" at a high level.
Lastly, the media may have variations in the type of data stored which causes the envelope to increase or decrease at different rates.
What is needed is adaptable threshold tracking which detects the peaks of the data stream and additionally can be easily upgraded, altered or changed while in use to be able to handle variability of media and of recording without artifically "hanging".